For the gaming industry, this seems to push Microsoft into 3rd place (by size) behind Sony and Tencent. So hardly a monopoly and akin to T-Mobile's acquisition of Sprint a few years ago. It makes Microsoft much more competitive against Sony and even Nintendo since it'll likely bolster their 1P offerings in the future.

But if Microsoft uses their ownership to favor their own game subscription services (aka GamePass) as well as platforms (aka Windows 11, Xbox console), then certainly that'll be monopolistic behavior. Interesting to note that they're probably #1-#2 in either of those sub-industries. It's possible to end up with an "Internet Explorer-esque" antitrust scenario if Microsoft removes or heavily discourages Activision and Bethesda from making their titles cross-platform.